This invention relates to the fabrication of heterostructure optical devices, such as semiconductor junction lasers, by molecular beam epitaxy (MBE).
In U.S. Pat. No. 3,615,931 granted to J. R. Authur, Jr. on Oct. 26, 1971 and assigned to the assignee hereof, there is described a comparatively new technique for the epitaxial growth of thin films of semiconductor materials, in which growth results from the simultaneous impingement of one or more molecular beams of the constituent elements onto a heated substrate. In particular the Arthur patent describes the basic MBE process for growing Group III(a)-V(a) thin films on a substrate preheated to a temperature within the range of about 450.degree.-650.degree. Centigrade and maintained at subatmospheric pressure. In U.S. Pat. No. 3,751,310 granted to A. Y. Cho on Aug. 7, 1973, there is described an MBE technique for doping such Group III(a)-V(a) thin films with Sn and Si, which act as donors, and with Ge, which is amphoteric depending on whether the growth surface structure is stabilized (i.e., rich) in the Group III(a) element (p-type) or the Group V(a) element (n-type). In addition, in copending application Ser. No. 310,209 (A. Y. Cho- M. B. Panish) filed on Nov. 29, 1972 (now U.S. Pat. No. 3,839,084) there is described a recent MBE technique for making Group III(a)-V(a) thin films p-type by doping with Mg.
One of the initial motivations for investigating MBE was the ability to fabricate thin epitaxial layers for optoelectronic devices because precise control of layer thickness and uniformity over a large area is readily achieved with MBE. Prior attempts to fabricate semiconductor devices, such as junction lasers, consistently met one recurring problem--anomalously high series resistance (e.g. 1000 ohms). In copending application Ser. No. 373,023 (A. Y. Cho-F. K. Reinhart), filed on June 25, 1973, relatively low series resistance (e.g., 2 ohms) in junction lasers and other devices has been achieved when one or more of the following steps are executed in the MBE technique: (1) on the substrate a high conductivity buffer layer is first grown having the same conductivity type as the substrate; (2) beginning with the buffer layer and until all semiconductor layers of the device are fabricated, the growth process is made to be continuous; and (3) the substrate is heated just prior to the growth of the high conductivity layer while a molecular beam of any element (e.g., arsenic) in the substrate having a relatively high vapor pressure impinges upon the substrate surface in order to suppress the loss of the element from the substrate.